1. Field of the Invention
The present invention relates to a stacked type electro-mechanical energy conversion element and a vibration wave driving apparatus of stacked structure consisting of a stack of a plurality of piezoelectric bodies and, more particularly, to a configuration for connecting electrodes between layers in the stacked type electro-mechanical energy conversion element.
2. Related Background Art
Piezoelectric elements having the electro-mechanical energy conversion function are used in various use applications. The piezoelectric elements are generally classified into the structure comprised of a single piezoelectric body of plate shape and the stacked structure comprised of multiple piezoelectric bodies of plate shape. The piezoelectric elements of the stacked structure can generate greater distortion with supply of a lower applied voltage than the piezoelectric elements of the structure comprised of only one piezoelectric body of plate shape.
A stacked type piezoelectric element is comprised of a plurality of piezoelectric layers of piezoelectric ceramics and electrode films (hereinafter referred to as internal electrodes) provided on surfaces of the respective piezoelectric layers. For connecting the internal electrodes on the respective piezoelectric layers to each other, it is common practice to provide electrode portions disposed on an outer peripheral surface or an inner peripheral surface of the stacked type piezoelectric element (hereinafter referred to as external electrodes), or to provide through holes along the stack direction in the piezoelectric layers and provide through electrodes (through holes) formed by burying an electrode material in the through holes.
FIGS. 5 and 6 show configurations of stacked type piezoelectric elements used in a vibration body of a rodlike vibration wave motor disclosed in U.S. Pat. No. 5,770,916.
The internal electrodes 103 indicated by hatching are formed on the surfaces of the second and lower piezoelectric layers 102 in the stacked type piezoelectric element 101 shown in FIG. 5. The internal electrodes 103 are not formed on the outer peripheral edges of the piezoelectric layers 102. In other words, the internal electrodes 103 are formed inside the outside diameter of the piezoelectric layers 102. Further, the internal electrodes 103 are out of contact with each other. Connection electrodes 103a (black solid portions in the drawing) are formed on the outer peripheral edges of the piezoelectric layers 102, and the connection electrodes 103a are in contact with the internal electrodes 103.
The internal electrodes 103 on the respective piezoelectric layers 102 are stacked so as to be aligned in identical phases, and the connection electrodes 103a are formed at identical positions on every other layer. Then the external electrodes 104 are formed at positions to be superimposed on the connection electrodes 103a, on the outer peripheral surface of the stacked piezoelectric element 101, so as to connect the connection electrodes 103a on every other layer. Namely, the internal electrodes 103 located in identical phases are arranged as electrically conductible on every other layer.
A plurality of surface electrodes 105 are provided along the circumferential direction and in the phases matched with those of the connection electrodes 103a, on the outer peripheral edge of the surface of the uppermost piezoelectric layer forming the stacked piezoelectric element 101. The surface electrodes 105 are connected to the external electrodes 104.
On the other hand, FIG. 6 shows another stacked piezoelectric element 201, in which the internal electrodes 203 are formed in structure similar to that shown in FIG. 5, on the surfaces of the piezoelectric layers 202 and in which the internal electrodes 203 are connected by the through electrodes (through holes) 204. The through electrodes 204 are exposed at their ends in the surface of the uppermost piezoelectric layer of the stacked piezoelectric element 201, thereby forming the surface electrodes 205.
Further, FIG. 7 shows an application example in which the aforementioned stacked piezoelectric element 101 of FIG. 5 is applied to the vibration body of the rodlike vibration wave motor. A wiring board 111 is kept in contact with the surface electrodes 105 of the stacked piezoelectric element 101, and the stacked piezoelectric element 101 and the wiring board 111 are placed between hollow metal members 21 and 22 of the vibration body. A bolt 23 penetrating a center hole of the stacked piezoelectric element 101 is inserted from the side of the metal member 22 to be screwed into the metal member 21. By tightening this bolt 23, the stacked piezoelectric element 101 and wiring board 111 are pinched and fixed between the two metal members 21 and 22. The wiring board 111 is connected to an unrepresented drive circuit and the drive circuit applies alternating voltages for driving, to the stacked piezoelectric element 101.
Likewise, in the case where the stacked piezoelectric element 201 shown in FIG. 6 is applied, the stacked piezoelectric element 201 and the wiring board 211 in contact with the surface electrodes 205 are also pinched and fixed between the metal members 21 and 22. The stacked piezoelectric element 201, which uses all the through electrodes as means for connecting the internal electrodes to each other, is incorporated into the vibration wave motor of the structure shown in FIG. 7, which is now under practical use as a driving source for driving a camera lens to effect autofocus.
The principle of driving of the rodlike vibration wave motor is as follows: a plurality of different bending vibrations with a temporal phase difference are generated in the vibration body equipped with the stacked piezoelectric element to force the distal end of the metal member 21 forming the vibration body, to perform a motion like a swinging motion. This motion rotates a rotor 24 kept in press contact with the metal member 21, through friction.
An example of the structure of the internal electrodes suitable for such driving is the quartered internal electrodes, as shown in FIGS. 5 and 6. Let us suppose that these internal electrodes are phase A, phase B, phase AG, and phase BG in the circumferential direction. Two internal electrodes located in the positional relation of 180xc2x0 (phase A and phase AG; and phase B and phase BG) are polarized in directions different from each other. These phases A to BG are formed so as to be identical among the second and lower piezoelectric layers, and the internal electrodes of identical phases on the different layers are electrically connected to each other by the aforementioned external electrodes or through electrodes.
When with the phases AG and BG being ground a high-frequency voltage (alternating signal) having a frequency approximately equal to the natural frequency of the vibration body is applied to phase A and to phase B different 90xc2x0 from the phase of phase A with a temporal phase difference between them, two bending vibrations perpendicular to each other are generated in the vibration body.
The method of interposing the stacked piezoelectric element 101, 201 and the wiring board 111, 211 between the metal members 21 and 22 as described above is high in reliability of electrical conduction between the stacked piezoelectric element 101, 201 and the wiring board 111, 211, and easy to assemble.
However, since the wiring board 111, 211 is interposed between the metal members forming the vibration body of the vibration wave motor, this wiring board 111, 211 causes damping of the vibrations. For this reason, there is conceivably plenty of scope for improvement in drive efficiency.
In addition, in the case of the stacked piezoelectric element 101 shown in FIG. 5, where the surface electrodes 105 are formed, for example, by inexpensive screen printing, the heights of the surface electrodes tend to become uneven. Therefore, more careful processing was needed.
The stacked piezoelectric element 201 shown in FIG. 6 required a system for forming the through electrodes and took a lot of processing time.
One of the features of the present invention is to provide a stacked type electro-mechanical energy conversion element comprising a stack of superimposed layers with an electro-mechanical energy conversion function having an electrode film formed on a superimposed surface, wherein part of the electrode film is connected to an electrode film formed on an edge portion of a layer,
wherein on a side face of the stacked electro-mechanical energy conversion element there are provided a connection terminal connectable to an external power supply, and a wiring portion connecting the connection terminal to the electrode film formed on the edge portion of the layer.
When this stacked electro-mechanical energy conversion element is applied to a vibration wave driving apparatus, the alternating signals can be applied from the side to the stacked electro-mechanical energy conversion element, which can obviate the need for interposing a circuit board between an elastic member forming an vibration body and the stacked electro-mechanical energy conversion element.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.